They already have holes built-in that, given the right sized driver, would require nothing but mounting the driver. Boom, instant enclosure. If I get it a little larger than the desired volume, I could fill the bottom with some sand or lead shot to "tune" the enclosure.

going to be too thin for good enclosures. The key is to get damping material in the middle of the sphere. Less of an issue if the motor is stuck right in the middle, but if there's nothing in the middle, a single frequency will dominate.

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"The dawn of Bose created the "Man Cave" and reduced testoterone levels worldwide by 18.5 per cent" Peteleoni

I did wonder about the 1/8" acrylic being too thin. But there may be other spherical options.

What about having a tennis ball suspended exactly in the middle of the enclosure?

Same issue- the difference from the outer surface to the surface of the tennis ball would be consistent all around. What you want is to vary this, or damp this mode. The tennis ball might raise the frequency and possible provide some damping but hard to say without testing.

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"The dawn of Bose created the "Man Cave" and reduced testoterone levels worldwide by 18.5 per cent" Peteleoni

Lighting fittings are a rich source of glass, acrylic and polycarbonate spheres. Most of them have a neck or rim around the opening which provides a good place to mount a driver or baffle. I have a selection of such spheres ranging from 6 inches to 2 feet in diameter , waiting for me to get a round tuit... They were very cheap, I found them at a junkyard.

The big polycarbonate ones are quite rigid, they are designed to withstand having rocks etc thrown at them - they are typically used in public spaces such as parks. I would think the main problem with such a sphere would be the driver's backwave mid and HF sound radiating out through the relatively thin surface. A coating of self-adhesive damping mat over the inside surface should reduce that. Or just pour in liquid damping material and swish it around until it hardens.

Another source of experimental enclosures is garden weed sprayers, the cheap pump-up ones with cylindrical tanks made of thick plastic. They come in sizes from about 3 litres to 8 litres. The tanks are thick walled and quite "dead" when rapped with knuckles. Cut an end off, screw in a round baffle board. Again, I have several in various sizes witing for my free time. They look ugly, but nothing a decorative sleeve won't fix. Check office supply stores for good-looking waste bins, and garden supply stores for decorative plant pot sleeves. Or just sew up a fabric pull-on sleeve.